JOVIANGENESIS

As the largest planet orbiting the sun, Jupiter has had a
profound influence on the solar system. But its origin remains a profound
mystery. To learn how Jupiter formed and how it has evolved, Juno will study
the gas giant’s gravitational andmagnetic fields, and explore the swirling
clouds that form Jupiter’s colorful, trademark atmosphere. The spacecraft
will also reveal what Jupiter is made of – and how much of it is water.

HISTORY OF THE SOLAR SYSTEM –THE MOTIVATION BEHIND THE JUNO MISSION

Four and a half billion years ago, a giant cloud of gas and dust, called a nebula, collapsed to form our solar system. Composed mainly of hydrogen gas, most of the nebula became the star we know as the Sun. The rest of the swirling cloud would condense to form earth and the other planets, asteroids and comets. It isn’t clear what triggered this collapse, but it does seem that whatever process produced our solar system is at work across the universe. We’ve observed half-formed stars – disks of gas in the midst of collapsing – and Jupiter-like planets orbiting other stars.

Jupiter was likely the first of the planets to form because it contains a lot of the same light gases that the sun is made of – hydrogen and helium. After the first few million years in the star’s life, it generated a wind that blew away most of the light gases that remained from the original nebula. For Jupiter to be primarily composed of hydrogen and helium, it must have formed while there was still a lot of those light gases around – when the solar system was young.

Since Jupiter is mainly made of the same stuff as the original nebula, the gas giant may hold clues about the origin of the solar system. As the nearest giant planet, studying Jupiter can also provide insight into planetary systems around other stars.

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What Led us to Juno?

When Galileo first saw Jupiter with his homemade telescope 400 years ago, he saw a dazzling planet with colorful clouds and bands, surrounded by its own moons like a miniature solar system. Ever since, we’ve been fascinated by Jupiter, snapping pictures with advanced telescopes and sending spacecraft to explore the gas giant. One spacecraft, named Galileo in honor of the scientist, spent eight years learning about Jupiter and its moons.

But we still don’t understand a few basic areas: we don’t know if the planet has a solid core; we don’t know how and where its magnetic field is produced; and we don’t know how much oxygen there is. Some theories about Jupiter’s formation predict that the planet’s oxygen weighs as much as 20 Earths. The abundance of oxygen isn’t just a major mystery about Jupiter, it’s the most important unanswered question about how our solar system formed.

As we’ve learned more about Jupiter, we’ve realized that we don’t understand planetary formation as well as we thought. In 1995, NASA’s Galileo spacecraft sent a probe into the thick clouds of Jupiter. From the information gleaned by the probe and from ideas developed over the years, we now know that Jupiter has a lot more heavy elements – that is, elements heavier than hydrogen and helium, such as nitrogen and carbon – than the sun. But if Jupiter formed from the same cloud of gas and dust as the sun, it should be made of the same stuff. How did it become so enriched with heavy elements?

We think that for Jupiter to be enriched, it must have been somehow assembled from many gas-containing icy bodies. But such objects would only have been able to exist far beyond Jupiter’s present orbit, where it would be much colder. The question, then, is whether Jupiter formed at its present orbit, somehow attracting these distant icy bodies, or whether Jupiter formed farther away from the Sun and then migrated inward.

In the last decade and a half, we’ve discovered hundreds of planets orbiting other stars. Many of these planets are gas giants bigger than Jupiter – and they are much closer to their stars than Jupiter is to the Sun. Why do these planetary systems seem so different from our own? We have to know how our own gas giants formed to under- stand whether our solar system formed in the same way as these other systems – or whether the solar system is a special case. Maybe the gas giants orbiting other stars weren’t born from the same processes as Jupiter. Maybe they’re failed stars – balls of gas that just aren’t massive enough for nuclear fusion to ignite.

One thing’s for sure, however. Although Jupiter’s colorful clouds get all the attention, the most enticing scientific mysteries are hiding deep inside the planet.

Why Jupiter?

There are many ideas as to how Jupiter might have formed. Some scientists think that it began as a solid chunk of heavy material, such as rock and ice. As its gravity gathered debris, it grew, increasing its gravitational pull. Eventually, it became so big that it had enough gravity to attract the light gases – hydrogen, for example – in the nebula around it.

Another possibility is that Jupiter formed when a small region of the gas disk that swirled around the young Sun suddenly collapsed on its own.

One of the most powerful aspects of science is its ability to make predictions about how the universe works. Various theories of Jupiter’s formation say different things about what the planet should be like. For example, one theory predicts that Jupiter should have a core of heavy, solid material – made out of elements like carbon, oxygen, nitrogen, and silicon – that’s as massive as three Earths. Another theory suggests that there should be nine Earths’ worth of material. Yet another idea says the core should weigh as much as 20 Earths. An entirely different group of theories makes predictions about how much of Jupiter is made of water.

By determining the nature of Jupiter’s core and how much water the planet has, scientists can narrow down the many ideas as to how Jupiter formed. Since Jupiter’s formation is inextricably tied to Earth’s, Juno’s mission is, in essence, about understanding our own origin.

We can’t observe Jupiter’s deep interior directly, but we can observe how the interior affects the space around the planet.

How do we study Jupiter's interior?

Juno is equipped with tools that allow us to learn about Jupiter’s interior – even if we can’t directly see inside the planet. Movement and density variations under the clouds – caused by a thick, churning blob of gas, for example – can subtly alter the gravitational field directly above the surface. By observing these slight effects, Juno can help deduce what’s inside. The spacecraft can also measure Jupiter’s magnetic field and, in doing so, determine the size of the solid core at the center – or whether there’s one at all.

JUNO AND THE PUBLIC

Juno should provide a giant leap in our understanding of Jupiter’s formation.

How does Juno explore Jupiter?

Even though several spacecraft have visited Jupiter, and
despite having the best telescopes on Earth and in space, there’s still a lot
we don’t know about the gas giant. We know that 99 percent of the planet is
hydrogen and helium, but the remaining one percent remains a mystery. We’re
also not sure whether there’s a solid core at the center or how Jupiter
generates its powerful magnetic field.

We’ve come up with many possible answers to these questions,
and many of our ideas are well supported by experiments and other space
missions. With the help of Juno, we will make dramatic progress in solving
these mysteries, allowing us to understand how Jupiter formed and became the
planet we know today.

THE SEARCH FOR WATER

One of the biggest questions we have about Jupiter is how
much of it is made of heavy elements – elements that are heavier than hydrogen
and helium. By far the most important heavy element is oxygen, whose most
common form is in water. Oxygen is the third most abundant element in the
universe, and we expect that it should account for more than half of Jupiter’s
heavy-element composition. In fact, the amount of oxygen in Jupiter could weigh
as much as 20 Earths.

PLUNGING INTO JUPITER’S DEPTHSAccording to data taken by spacecraft and telescopes,
Jupiter must be made of materials heavier than hydrogen and helium. Most
theories about what Jupiter looks like inside suggest that there’s a solid core
at its center. But so far, we have never been able to verify its existence.
We’re also unsure whether the core is like a solid ball with a surface, or
whether the core’s interface with the rest of the planet is more gradual, with
Jupiter’s interior gas becoming denser until it becomes solid at the center.

Different theories, or models, of how Jupiter formed make
different predictions about the size, mass, and composition of the core. In
some models, Jupiter’s core could weigh as much as three, nine, or even twenty
Earths. By determining what Jupiter’s core is like, Juno will help us narrow
down the correct model. And if Juno finds that the core is nothing like what we
expected, then we would be forced to rethink our ideas about how giant planets
like Jupiter form.

Another mystery is the structure of Jupiter’s swirling
clouds, bands, and storms. Jupiter’s most breathtaking surface features, like
its Great Red Spot, could be connected to the structure and motions of gas deep
in its interior. Or, they could be shallow patterns on the outermost layer of
the atmosphere, like drops of oil on a pool of water. With Juno, we will be
able to see the structure and movement of material deep beneath Jupiter’s
clouds for the first time.

EXPLORING THE MAGNETIC FIELDDeep inside Jupiter, the crushing weight of the planet
creates extreme temperatures and pressures. Researchers have recreated similar
conditions in the laboratory – but only for mere fractions of a second. Their
experiments suggest that at some point inside Jupiter, maybe about a third to a
half of the way toward its center, the pressure and temperature become so
intense that the planet’s hydrogen gas turns into liquid and conducts
electricity. We think that it’s here, where hydrogen takes on this exotic form,
that Jupiter’s huge magnetic field is produced.

But the conditions here are so strange that we don’t have a
good understanding of what exactly goes on. The magnetic field could be
generated in a way that’s similar to how Earth’s field is generated. Or, the
engine behind Jupiter’s magnetic field could more closely resemble how material
flows inside the Sun. To improve our understanding of Jupiter’s magnetic field,
Juno will map the field and monitor how it changes over time.

As tiny, charged particles fly through space, they get caught
up in Jupiter’s magnetic field, which channels them toward the planet’s north
and south poles. When these particles slam into the poles, they create intense
light shows – Jupiter’s aurorae, northern and southern lights just like on
Earth. Juno is equipped with powerful instruments that can measure the aurorae
and detect these particles as they stream past. These processes also produce
radio signals that Juno can listen to with its radio antennas. The data will
help us understand the complex interactions between Jupiter’s rotation, its
atmosphere and its magnetic field.

What's in a name?

In Roman mythology, Jupiter, king of the gods, shrouded
himself with clouds to hide his mischief. But Jupiter’s wife, the goddess Juno,
was able to peer through the clouds and discover the truth. Likewise, the Juno
spacecraft will look beneath Jupiter’s clouds to help us understand the
planet’s structure and history.

The journey to Jupiter

The Galileo Probe

When the Galileo probe plunged into Jupiter’s clouds, it discovered something unexpected. It found that there were two to three times more volatile substances – materials that melt at low temperatures, such as the elements argon, krypton, xenon, carbon, and nitrogen – on Jupiter than on the Sun.

These elements could have come from small, asteroid- and comet-like bodies. For these bodies to contain the argon and nitrogen that’s found in Jupiter, they must have formed at temperatures lower than 30° Kelvin (-405 Fahrenheit). This means that either Jupiter was born far from the Sun, where it would have been cold enough for these elements to exist, and then migrated closer, or that comets and asteroids brought these elements with them when they crashed into Jupiter.

But there’s another puzzle. The Galileo probe also found that Jupiter has much less oxygen than the sun. Comets, which were leftover objects in the cold regions of the solar system that failed to grow large enough to become planets, have a lot of water. Oxygen is a main component of water, and if these comets delivered elements to Jupiter, why didn’t they also bring oxygen? One possibility is that the lack of oxygen is just an anomaly; the probe entered a “hot spot” on Jupiter, a region that just happened to be dry, like a desert. Or, the lack of water and oxygen could be a fundamental clue about the formation of gas giants.

By getting a more detailed measurement of Jupiter’s heavy elements with Juno, scientists will be able to determine how the planet became chemically enriched. If these elements were brought by comets from a distant, icy region of the solar system called the Kuiper Belt, then Jupiter should have roughly the equal amounts of oxygen and heavy elements. But if Juno finds that there’s more oxygen than heavy elements, then these elements must have originated locally, near Jupiter’s present location.

Where is Juno now?

Juno is currently headed back toward the inner solar system for a planned Earth flyby gravity assist maneuver on Oct. 9, 2013. The Juno mission operations team is continuing their planning activities in advance of this critical maneuver. The gravity assist will give the spacecraft the boost it needs to reach Jupiter, where it is slated to arrive in July 2016.